1. Field of the Invention
The present invention relates to an epitaxial wafer, and more particularly, to a gallium aluminum arsenide (GaAlAs) mixed crystalline epitaxial wafer suitable for the production of a high luminance light emitting diode (LED).
2. Description of the Related Art
Since LED's can be driven by a low voltage and have a long life, they are widely used as light emitting indicators; in particularly, they are now used as outdoor light emitting indicators such as a destination indicating plate or a traffic signal, among others. Such devices require an LED, to provide large light output power to ensure that they can function as indicators as required even under sunny conditions.
An example of an LED having a large output power, is that produced by using, for example, a GaAlAs mixed crystalline epitaxial wafer, and this kind of LED using a GaAlAs mixed crystal is divided into a single hetero type epitaxial wafer and a double hetero type epitaxial wafer as is well known.
The single hetero type epitaxial wafer is produced by the steps of forming an n-type GaAlAs mixed crystalline layer having an indirect transition type band structure or a single crystalline substrate and forming a p type GaAlAs mixed crystalline layer having a direct transition type band structure on the n type GaAlAs mixed layer.
As shown in FIG. 1, in the single hetero type epitaxial wafer, a rapid change of the AlAs mixed crystal ratio (hereinafter explained in detail) occurs at depth A, i.e., at a pn junction portion of the wafer.
The double hetero type epitaxial wafer is formed of a GaAlAs mixed layer having n type and p type indirect transition type GaAlAs mixed layers on a single crystalline substrate and an active layer consisting of a direct transition type p type GaAlAs mixed crystalline layer having a thickness of about 0.5 to 5 .mu.m formed therebetween.
As shown in FIG. 2, a steep change of the AlAs mixed crystal ratio occurs at depths B and C, i.e., at the hetero junction The region B to C denotes the direct transition type GaAlAs mixed crystalline layer. This rapid change of the mixed crystal ratio is used to increase the efficiency of carrier injection and carrier recombination. The single hetero epitaxial wafer can be easily produced, as it can be formed merely by growing two kinds of mixed crystalline layers as explained above, and the thickness of each mixed crystalline layer can be roughly controlled, but LED's fabricated from the single hetero epitaxial wafer have low light output power. On the other hand, although LED's fabricated from the double hetero type epitaxial wafer have high output power, three kind of GaAlAs mixed crystalline layer must be formed thereon: Particularly, the thickness of the p type active layer between the n and p type indirect transition type GaAlAs mixed crystalline layers must be precisely controlled, and thus the growth processes for the double hetero type epitaxial wafer is very complicated.